Chiral rhodium complexes covalently anchored on carbon nanotubes for enantioselective hydrogenation

Anisotropic nanomaterials for asymmetric synthesis

The production of enantiopure chemicals is an essential part of modern chemical industry. Hence, the emergence of asymmetric catalysis led to dramatic changes in the procedures of chemical synthesis, and now it provides the most advantageous and economically executable solution for large-scale production of chiral chemicals. In recent years, nanostructures have emerged as potential materials for asymmetric synthesis. Indeed, on the one hand, nanomaterials offer great opportunities as catalysts in asymmetric catalysis, due to their tunable absorption, chirality, and unique energy transfer properties; on the other hand, the advantages of the larger surface area, increased number of unsaturated coordination centres, and more accessible active sites open prospects for catalyst encapsulation, partial or complete, in a nanoscale cavity, pore, pocket, or channel leading to alteration of the chemical reactivity through spatial confinement. This review focuses on anisotropic nanomaterials and considers the state-of-the-art progress in asymmetric synthesis catalysed by 1D, 2D and 3D nanostructures. The discussion comprises three main sections according to the nanostructure dimensionality. We analyze recent advances in materials and structure development, discuss the functional role of the nanomaterials in asymmetric synthesis, chirality, confinement effects, and reported enantioselectivity. Finally, the new opportunities and challenges of anisotropic 1D, 2D, and 3D nanomaterials in asymmetric synthesis, as well as the future prospects and current trends of the design and applications of these materials are analyzed in the Conclusions and outlook section.

Mesostructuring layered materials: self-supported mesoporous layered double hydroxide nanotubes

Synthesis of layered materials exhibiting hierarchical porosity remains challenging, but nevertheless worthwhile because it turns such solids into functional materials with high specific surface area. Using a soft-templating strategy in combination with the incorporation of 8-fold coordinated Eu3+, self-assembly of self-supported layered double hydroxide (LDH) nanotubes has been achieved. Heteromorphic equimolar substitution of Al3+ by Eu3+ in Zn2+/Al3+ LDH solids intercalated with 1,3,5-benzenetricarboxylate anions (BTC) assists precipitation of the double hydroxide layers onto the convex surface of Pluronic® P-123 worm-like micelles, yielding multilayer cylinders of BTC-intercalated LDHs. Removal of the micellar template is easily achieved by liquid extraction with methanol, yielding a network of interconnected, well-defined, self-supported, multi-walled, hollow cylindrical nanotubes. Removal of Eu3+ from the synthesis disables formation of the nanotubular morphology, but still yields LDHs containing a network of embedded mesopores, resulting in a specific surface area that is 5-fold higher as compared to standard LDHs.

Construction of Zn(II) Linear Trinuclear Secondary Building Units from A Coordination Polymer Based on α-Acetamidocinnamic Acid and 4-Phenylpyridine

The synthesis and characterization of one coordination polymer and two trinuclear complexes are presented. The coordination polymer [Zn2(µ-O,O'-ACA)2(ACA)2(4-Phpy)2]n (1) has been obtained by the reaction between Zn(OAc)2·2H2O, α-acetamidocinnamic acid (HACA), and 4-phenylpyridine (4-Phpy) using EtOH as solvent. Its recrystallization in CH3CN or EtOH yields two trinuclear complexes, both having pinwheel arrays with formulas [Zn3(µ-ACA)6(4-Phpy)2]·4CH3CN (2·4CH3CN) and [Zn3(µ-ACA)6(EtOH)2]·4EtOH (3·4EtOH), respectively. These trinuclear species, unavoidably lose their solvent co-crystallized molecules at RT yielding the complexes [Zn3(µ-ACA)6(4-Phpy)2] (2) and [Zn3(µ-ACA)6(EtOH)2] (3). In addition, compound 2 has also been obtained reacting Zn(OAc)2·2H2O, HACA, and 4-Phpy in a 1:2:2 ratio using CH3CN as solvent. Compounds 1-3 have been characterized by analytical and spectroscopic techniques. Furthermore, single crystals suitable for X-ray diffraction method for compounds 1, 2·4CH3CN, and 3·4EtOH were obtained and their supramolecular interactions have been studied and discussed, showing 2D supramolecular planes for the trinuclear complexes and a 3D supramolecular network for the coordination polymer. Finally, the supramolecular interactions of 2·4CH3CN and 3·4EtOH have been compared using Hirshfeld surface analysis and electrostatic potential calculations.

Ruthenium anchored on carbon nanotube electrocatalyst for hydrogen production with enhanced Faradaic efficiency

Developing efficient and stable electrocatalysts is crucial for the electrochemical production of pure and clean hydrogen. For practical applications, an economical and facile method of producing catalysts for the hydrogen evolution reaction (HER) is essential. Here, we report ruthenium (Ru) nanoparticles uniformly deposited on multi-walled carbon nanotubes (MWCNTs) as an efficient HER catalyst. The catalyst exhibits the small overpotentials of 13 and 17 mV at a current density of 10 mA cm-2 in 0.5 M aq. H2SO4 and 1.0 M aq. KOH, respectively, surpassing the commercial Pt/C (16 mV and 33 mV). Moreover, the catalyst has excellent stability in both media, showing almost "zeroloss" during cycling. In a real device, the catalyst produces 15.4% more hydrogen per power consumed, and shows a higher Faradaic efficiency (92.28%) than the benchmark Pt/C (85.97%). Density functional theory calculations suggest that Ru-C bonding is the most plausible active site for the HER.

Synthesis and characterization of a supported Pd complex on carbon nanofibers for the selective decarbonylation of stearic acid to 1-heptadecene: the importance of subnanometric Pd dispersion

Evaluation of the dispersion of Pd active sites on the catalyst performance during fatty acids decarbonylation to α-olefins was explored in this work. Pd subnanometric particles, clusters and aggregates were found to modulate the catalyst activity.

Experimental and theoretical insights into the synthesis of α-aminoalkyl naphthol derivatives catalyzed by a manganese complex immobilized on multi-walled carbon nanotubes

The full details of the synthesis of α-aminoalkyl naphthol derivatives promoted by a manganese complex immobilized on multi-walled carbon nanotubes (CNTs@Mn-bpy) are described, with a particular focus on theoretical mechanistic aspects. After characterization studies on the nanocatalyst prepared from a homogeneous manganese complex, we optimized the catalytic reaction conditions, and the heterogeneous nanocatalyst showed important features such as higher efficiency and reusability compared to other reported catalysts. α-Aminoalkyl naphthol derivatives were obtained in excellent chemical yields and short reaction times and with high turnover numbers. In order to better understand the effect of CNT@Mn-bpy as a nanocatalyst on the reaction mechanism, the final step of the described synthesis has been investigated using density functional theory (DFT). Compared to the uncatalyzed reaction, the activation energy of the catalyzed reaction is lowered by 34.07 kcal mol^-1. Also, our theoretical calculation shows that if the Mn metal of CNT@Mn-bpy is replaced with the Cu metal, the catalytic effect of this nanocatalyst on the activation energy of the studied reaction was significantly decreased.

Visible light assisted hydrogen generation from complete decomposition of hydrous hydrazine using rhodium modified TiO2 photocatalysts

Hydrogen is considered to be an ideal energy carrier, which produces only water when combined with oxygen and thus has no detrimental effect on the environment. While the catalytic decomposition of hydrous hydrazine for the production of hydrogen is well explored, little is known about its photocatalytic decomposition. The present paper describes a highly efficient photochemical methodology for the production of hydrogen through the decomposition of aqueous hydrazine using titanium dioxide nanoparticles modified with a Rh(i) coordinated catechol phosphane ligand (TiO2-Rh) as a photocatalyst under visible light irradiation. After 12 h of visible light irradiation, the hydrogen yield was 413 μmol g-1 cat with a hydrogen evolution rate of 34.4 μmol g-1 cat h-1. Unmodified TiO2 nanoparticles offered a hydrogen yield of 83 μmol g-1 cat and a hydrogen evolution rate of only 6.9 μmol g-1 cat h-1. The developed photocatalyst was robust under the experimental conditions and could be efficiently reused for five subsequent runs without any significant change in its activity. The higher stability of the photocatalyst is attributed to the covalent attachment of the Rh complex, whereas the higher activity is believed to be due to the synergistic mechanism that resulted in better electron transfer from the Rh complex to the conduction band of TiO2.

Hierarchical self-supported ZnAlEu LDH nanotubes hosting luminescent CdTe quantum dots

Self-assembled, cylindrical LDH nanotubes (∅ 20nm) host large species such as nanodots or bulky substrates in their central cavity, leaving the interlayer space available for anionic sensitizer molecules.

Nickel(ii) complex covalently anchored on core shell structured SiO2@Fe3O4 nanoparticles: a robust and magnetically retrievable catalyst for direct one-pot reductive amination of ketones

A magnetic silica based nickel nanocatalyst has been fabricated for the direct one-pot reductive amination of ketones.

Carbon nanotubes and catalysis: the many facets of a successful marriage

Carbon nanotubes have emerged as unique carbon allotropes that bear very interesting prospects in catalysis.

The advantages of covalently attaching organometallic catalysts to a carbon black support: recyclable Rh(i) complexes that deliver enhanced conversion and product selectivity

A bidentate Rh(i) coordination complex was covalently anchored to a carbon black support. The resultant hybrid catalyst was found to be active for both hydroamination and dihydroalkoxylation reactions and was readily recycled.

Phosphole-based ligands in catalysis

This review provides an overview of phosphole-based ligand families (monophospholes, multidentate hybrid phosphole ligands, diphosphole and 2,2′-biphosphole-based ligands) and their potential in metal- and organo-catalyzed reactions (asymmetric reactions included).